The present invention relates to video chroma keyers, and more particularly to an improved chroma keyer with a secondary hue selector for chrominance suppression of "blue fringing" effects resulting from incomplete chrominance suppression of a backing color around the edges of an object in a foreground video video.
In chroma keying operations an object is presented in a studio set, with the studio set color providing a specific background hue. A hue selector circuit detects the specific color of the set, or backing, and creates a mixing, or chroma key, signal based on the presence or absence of this color in the foreground video signal. When the color is present, the foreground video signal is suppressed, and a second background scene is substituted to present the illusion of the foreground object being in the same scene as the substituted background. For example a weatherman may actually stand in front of a studio set having a blue or green backing, but appear to be standing in front of an animated weather map.
One basic form of a chroma keyer is to mix the foreground and background signals according to the following equation: EQU Composite=(FG.times.(1-SH)+(BG.times.SH))
where Composite is the resulting video signal, FG is the foreground video signal, BG is the background video signal and SH is the chroma key signal, or the amount of selected hue present. This defines a multiplicative keyer such as is disclosed in U.S. Pat. No. 4,488,169 issued Dec. 11, 1984 to Yamamoto entitled "Digital Chromakey Apparatus."
A second basic form of a chroma keyer alters the foreground video by subtracting the chrominance and luminance of the backing color from the foreground video. A matte generator is set to the same hue as the hue selected for the backing color. The luminance of the matte generator is set to zero so that the output is only a chrominance signal without luminance. This matte color is then multiplied by the chroma key signal derived from the foreground video according to the hue from a hue selector. The resulting modulated matte color is subtracted from the foreground video. In the case of a composite system the subtraction is performed by using vector subtraction, while in the case of a component system the subtraction is performed directly on the appropriate component signal. The resulting modified foreground video signal presents the foreground object over a gray background. A second subtraction of a luminance signal, also derived from the hue selector, from the modified foreground video signal produces a shaped foreground video signal with the foreground object presented over black.
Once the shaped foreground signal has been created in this manner, the background signal is keyed in identically as in the multiplicative keyer according to the equation: EQU Composite=MFG+(BG.times.SH)
where MFG is the shaped foreground signal with chrominance and luminance around the foreground object suppressed. This second technique has the advantage of not multiplying the foreground video by the (1-key) signal. An example of this subtractive technique is disclosed in U.S. Pat. No. 4,409,611 issued Oct. 11, 1983 to Vlahos entitled "Encoded Signal Color Image Compositing."
Chroma keyer systems may be in either component, where luminance and chrominance are carried in separate channels, or composite, where the chrominance is encoded onto the luminance, implementations. In either case the hue selector is preferably driven with a component version of the foreground video signal so that full bandwidth chrominance information is used. In the event that the foreground video is available only in composite form, a decoder is included to produce the component version of the foreground video signal.
A fundamental problem with these techniques is "blue fringing", where in the common case of a blue backing incomplete suppression of the foreground video signal occurs around the edges of the foreground object. This occurs for two reasons.
First the frequency response of a camera and associated equipment is limited, so that where the edge of the foreground object meets the backing color, a soft transition between the foreground object color and the backing color occurs. Where this transition is taking place, the hue selector no longer recognizes the hue, or color, as being the selected hue, or backing color, so that the foreground video signal is selected even though the foreground video signal still contains a fairly high content of the backing color. This results in "blue fringing" around the foreground object when a blue backing is used.
Second if a foreground object is presented with hair, fur, transparency or translucency, the backing color shines through or reflects partially off the object, effectively mixing the color of the foreground object with the backing color. For example, if a blonde-haired performer is presented in front of a blue backing, the yellow hue of fringes of the hair is mixed with the blue color of the backing to create a halo of hair with a greenish cast. In this case the hue selector should not key out this hair, since that would present an unnatural shape to the performer's head. When the performer is keyed into a background scene without any blue content, the eye detects the discolored fringe as having a noticeably blue appearance.
In the prior art, blue fringing effects have been dealt with by changing the shape of the chroma key signal with a mask. But the mask is arbitrary and must be established manually.
What is desired is an improved chroma keyer for suppressing the blue fringing effect automatically without masking.